The present invention relates to a method and apparatus for cardiac pacing and, more particularly, for Atrial-His-Ventricular sequential pacing to improve sinoatrial node dysfunction or heart block superior to the His bundle. As a derivative, His-Ventricular sequential pacing can be employed to treat permanent atrial fibrillation.
The sinoatrial (SA) node represents the natural pacemaker that controls the rhythmic electrical excitation in a normal human heart. At an appropriate time, an electrical impulse arising from the SA node is transmitted to the right and left atrial chambers. This impulse causes muscle tissue surrounding the atrium to depolarize and contract which generates an electrical signal known as a P-wave. The same electrical impulse arising from the SA node also travels to the right and left ventricles through the atrioventricular (AV) node and atrioventricular (AV) bundle. The AV node, situated in the lower portion of the right atrium, receives the impulse to contract. The impulse is then transmitted through the AV bundle comprising Common Bundle of His (His bundle), right and left bundle branches, and Purkinje fibers that cover most of the endocardial surface of the ventricles. The ventricular muscle tissue depolarizes, generating an R-wave, then contracts. This forces blood held in the ventricles through the arteries and to various body locations. This action is repeated in a rhythmic cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill.
Disturbances of impulse formation by the sinus node and/or AV conduction block due to disease and aging are commonly treated by artificial pacing. An artificial pacemaker is an implantable medical device that monitors the activity of the heart for the occurrence of P- and/or R-waves. When a P- or R-wave is not sensed after a prescribed period of time, the pacemaker electronically generates stimuli in order to force the depolarization of the atria and/or ventricles. A pacemaker-generated stimulus that is delivered to the atria is known as an A-pulse, whereas a stimulus delivered to the ventricles is a V-pulse.
Different methods of artificial pacing have been employed including single or bi-ventricular pacing and dual chamber pacing. In single ventricular pacing, a pacing lead connected to an electrode is typically implanted in the apex of the right ventricle to deliver electrical impulses to the ventricular muscle tissue. However, this type of pacing results in the loss of synchronous mechanical contraction of the right and left ventricles due to the interventricular delay in impulse propagation to the left ventricle. This results in an immediate decrease in cardiac output along with potential deterioration of ventricular function over the long term as permanent changes occur in myocardial perfusion and structure. A method to pace both ventricles, bi-ventricular pacing, has been demonstrated to restore substantially simultaneous contraction of both ventricles and is accomplished by placing one pacing lead in the apex of the right ventricle and another pacing lead through the coronary sinus into a vein on the left ventricular wall. The surgical procedure used to implant the pacing lead in the coronary sinus however may be complex and a long pacing lead is needed to connect the electrode to the pulse generator thus requiring a higher voltage resulting in a large drain on the power source generating the pulses. In dual chamber pacing (DDD/R), electrodes connected to pacing leads are placed in the atria (for example the right atrium) and one or both of the ventricles. Under this method, ventricular synchrony can be achieved when electrodes are placed in both ventricles and an optimal delay between the A pulse and V pulse (AV delay) is utilized. However, this still does not produce a similarly coordinated contraction as compared to natural AV bundle activation.
Patients with SA node dysfunction or heart block superior to the His bundle, due to their conditions, suffer from a delay in the electrical response from the atria to the ventricles. According to current practice, patients with these conditions typically receive a DDD/R pacemaker. Many of these patients however still have normal ventricular contraction and thus are unnecessarily ventricularly paced due to pacemaker programming restrictions and/or prolonged AV conduction times. Because of this unnecessary ventricular pacing, these patients experience a decrease in cardiac efficiency due to the uncoordinated contraction sequence and eventually exhibit adverse long-term effects. Therefore, it""s desired to develop a pacing system for these patients, that is more tailored to their cardiac condition, without utilizing ventricular pacing means.
It has recently been shown in U.S. Pat. No. 5,320,642 (Scherlag) that a pacing lead can be implanted near the His bundle, just below the AV node to pace the ventricles as an alternative to ventricular pacing. When the ventricles are paced through this natural conduction system, the ventricles contract in a more coordinated fashion as compared to pacing the ventricles themselves thus improving cardiac output. Also, patients receiving ventricular pacing who do not need to be ventricularly paced are spared from the long term harmful hemodynamic effects that occur from continuous ventricular pacing. However, there is a risk that the electrode may become dislodged from the His bundle or that the patient may subsequently experience heart block inferior to the His bundle. Therefore, it""s desired to utilize this more natural His bundle pacing system in order to obtain a more coordinated contraction sequence, yet retain the option to ventricularly pace should the His bundle pacing lead fail or heart block below the His bundle occur.
The present invention is directed to a pacing apparatus and method for sequentially pacing the atria, His bundle and ventricles to provide synchronous mechanical contractions to improve cardiac output and prevent long term hemodynamic effects caused by unnecessary ventricular pacing. The pacing apparatus includes pacing leads extending from a pacer that lead to prescribed positions in the atria, AV septum and ventricle of the heart. The pacing leads have electrodes attached to their distal ends for measuring and delivering electrical impulses. The pacing leads leading to the atria and ventricles are programmed to deliver electrical pulses, on a demand mode basis, if natural electrical signals are not measured within a predetermined period of time. The pacing lead leading to the AV septum delivers continuous electrical pulses to the His bundle immediately following the sensing of atrial activity. These electrical pulses then travel through the right and left bundle branches and to the Purkinje fibers causing the ventricles to depolarize and contract. By utilizing this natural conduction system, the ventricles contract in a more coordinated manner as compared to ventricular pacing thus improving cardiac output and performance.
In one embodiment of the present invention, the pacing apparatus is provided for patients who suffer from sino-atrial (SA) node dysfunction or heart block superior to the His bundle. Because these patients typically have normal ventricular contraction, continuously pacing the His bundle results in electrical pulses being sent from the atria to the ventricles to travel more quickly thus allowing the ventricular pacing lead to remain dormant during the pacing cycle. The ventricular pacing is provided only for emergency situations such as, for example, inter alia, if the His bundle pacing lead should fail or if heart block inferior to the His bundle should occur.
In another embodiment of the present invention, the pacing apparatus is provided for patients who suffer from permanent atrial fibrillation. The His bundle pacing lead is implanted following AV node ablation to provide electrical pulses to the ventricles causing depolarization and contraction. Again, a ventricular pacing lead is provided but remains dormant and is activated only in emergency situations.